1. Field of the Invention
The present invention relates generally to endodontic instruments and, more particularly, to root canal files or reamers used in the cleaning of material present in the root canal of a human tooth and for enlarging and shaping the root canal so that it may be prepared for filling.
2. Description of the Prior Art
A relatively common but difficult dental procedure is the cleaning and filling of the root canal of a patient's tooth. In the performance of a root canal procedure, a hole is first cut in the crown or exposed portion of the tooth, typically ether in the biting surface of the tooth, for posterior teeth or in the side of the tooth on the interior of the jaw for incisor teeth. Small endodontic instruments known generally as root canal files are then used to clean out the material present in the root canal, and to shape the root canal so that a tapered filling material may be inserted into the root canal to fill it. An example of such an instrument, also called a broach, is shown in U.S. Pat. No. 250,544.
Two types of instruments are in general use as root canal files, namely the Hedstrom instrument and the K-type instrument. The K-type instrument is an axially twisted, tapered, triangular or square cross-sectional shaft which has three or four spiral cutting edges along the tapered shaft and a conical tapered tip on the end thereof. The Hedstrom-type instrument is a lathe-cut file having a round tapered shaft with one or two spiral cutting flutes machined into the shaft all the way to the tip. The main difference resulting from the construction of the two types of files is that the K-type file will cut in either rotational direction, or when moved up and down, while the Hedstrom-type file will cut only when moved up and down in the root canal.
When a root canal is being cleaned and shaped, a series of files having increasing diameters is used to gradually enlarge the root canal. The files are held between the thumb and forefinger of one hand by the dentist. Each file in a set of the known prior art has an identical taper from one end to the other. For example, in a typical K-type file set the taper is 0.32 millimeters on ever file, with the files coming in a number of sizes. The size number charcterizing the file is the diameter of the file at the tip in hundredths of a millimeter, and the diameter of th file at the large end is thus 0.32 millimeters greater than this tip diameter. A complete set will include sizes 06, 08, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 80, 90, 100, 110, and 120, while sizes 8-60 will typically be used. Hedstrom-type instruments similarly come in sets of increasing size, typically from 0.10 to 1.40 millimeter tip size, with 0.15-0.60 millimeter tip sizes typically being used.
Unlike the files, root canals are seldom straight. If the files which are used deviate substantially from the original curvature of the root canal, the tooth may be irreversibly damaged. If the file is too straight and cuts through the side of the root, which is referred to as a perforation of the root, the tooth must then be removed. If the tip of the file does not follow the curvature of the canal and bores a passage branching out from the root canal, which is referred to as ledging, surgical correction of the problem is necessitated.
It is therefore apparent that the files must be bent prior to insertion into the root canal and use of the files to prepare the root canal. Such bending of the files is preferably done by the instrument described in my U.S. patent application Ser. No. 899,342, entitled "Endodontic Root Canal File Bending Pliers", and filed concurrently with the present application, and that application is hereby incorporated herein by reference. While the files having smaller diameters may be easily bent, the larger diameter files do not bend easily.
In addition to bending the file, it is necessary to pull a precurved file along the outside of the canal curvature when shaping the canal to avoid perforating the root. This technique is called an anti-curvature motion, is difficult to teach to dentists, and is time-consuming for even the most skilled practitioners to perform.
It is thus apparent that the art of root canal filing is one which requires great skill to prevent damage to the tooth. The technique used with a set of files having an identical taper to clean and shape the root canal is referred to as the "step-back" technique, with each successively larger file being used further back from the end of the canal. This technique is, at best, a difficult and time-consuming method.
A slightly different file is disclosed in U.S. Pat. No. 4,536,159, to Roane. The Roane file rounds the cutting edges near the tip of the file, in an attempt to prevent the file from ledging the root canal. While the Roane file may reduce or eliminate occurrences of ledging, it will not do anything about the more serious problem of perforating the curved roots of teeth.
Other types of files have been developed for similar purposes. The U.S. Pat. No. 4,260,379 of Graves et al discloses a file of rhomboidal cross section in which a tapered blade is ground to develop an oblique parallelogram and twisted. McSpadden U.S. Pat. No. 4,299,571 discloses a file with a blunt tip with approximately 3 mm of non-cutting file provided adjacent the end. U.S. Pat. No. 4,332,561, also of McSpadden, combines the pilot tip of the earlier McSpadden patent with double-fluted design to develop a Hedstrom-type file.
It is therefore apparent that it would be desirable to have a file design which could eliminate or reduce the instances of both ledging and perforation. Achieving these objectives is the main object of the present invention. In addition, it is an object of the present invention to minimize the time and effort necessary to clean and shape the root canal. Finally, the present invention also seeks to reduce the level of training and skill necessary to properly perform root canal operations.